A Washington consultant told Arlington County school officials last night that the system's senior high school enrollment could drop as much as 60 percent over the next decade. But the consultant said that overall enrollment could stabilize.

The projections were part of a report on Arlington demographics to school board members who must decide the number and location of schools that may be closed next year in the wake of declining enrollment.

William H. Wilken, a partner in the educational research firm of Helminski & Wilken, said that although the county's senior high schools will experience "drastic losses," declines in enrollment at the intermediate level will be moderate and elementary schools may see a small increase by 1984.

Overall enrollment could increase or decrease by 1,300 to 1,500 students, Wilken said, from the current population of 14,500 students. The county has lost approximately 50 percent of its student population over the last 20 years. However, Wilken said his findings are based on current enrollment declines that have been moderate in recent years.

Wilken also said sections of the county serving elementary schools -- such as Tuckahoe, Abingdon, Drew, Ashlawn and Barrett -- may show greater losses in the number of elementary school-age youngsters residing there. Similar data affecting senior and junior high schools was not available.

School officials said they were not surprised by the findings but reserved comment until they could study the report further. School Superintendent Charles Nunley said the report reflects data school officials "have been saying all along."

Wilken said the findings are based on factors including the economy, housing, birth rates, the Orange Metro line, and school promotion policies. The report is to be distributed to all school principals, officials said.